Embodiments of the present invention relate to encoding data signals for ultrasonic communication through a multipath fading channel.
Ultrasound technology has been developed for practical applications such measuring fluid velocity in a pipe, measuring characteristics of a pipe and of fluid in the pipe, ultrasonic communication through various media such as metal pipes, underwater acoustic measurements, medical applications, and numerous other applications.
Referring to FIG. 1, for example, Hosman et al., “Design and Characterization of an MFSK-Based Transmitter/Receiver for Ultrasonic Communication Through Metallic Structures,” IEEE Trans. On Instrumentation and Measurement, Vol. 60, No. 12, pp. 3767-3774 (December 2011) disclose an ultrasonic communication system for communicating through steel corner posts of shipping containers, which is incorporated herein by reference in its entirety. The communication system transmits multitone frequency-shift keyed (MFSK) data through the steel corner posts by means of attached ultrasonic transducers to a receiver external to the shipping containers. The steel corner posts are characterized as a metal multipath fading channel. Hosman et al. employ an MFSK encoding system as shown at FIG. 2. A data word is applied to a symbol encoder to selectively apply frequencies f0 through fN-1 to a sum circuit. The selected frequencies are then applied to an ultrasonic transducer for transmission through the steel corner posts. The MFSK system of Hosman et al. uses different combinations of Q summed tones from N available tones to produce
      (                            N                                      Q                      )     MFSK symbols, where
      (                            N                                      Q                      )     is defined as N!/(Q!(N−Q)!). MFSK encoding advantageously produces substantially more encoded symbols than traditional frequency shift keyed (FSK) encoding as shown at FIG. 3. Here, for example, N=32 available tones will encode 5 bits (log2 (32)) in each FSK symbol. By way of comparison, MFSK will encode 29 bits or the integer portion of log2
      (                            N                                      Q                      )     for Q=16. This results in a significantly higher data rate for MFSK encoding with the same number of available tones. The present inventors have realized a need to improve communication techniques through the multipath fading channel to further improve data throughput and reduce symbol error rate (SER). Accordingly, the preferred embodiments described below are directed toward improving upon the prior art.